1. Field of the Invention
The present invention relates to an image forming method and apparatus, with a multi-beam light source, capable of changing a writing density. More particularly, the present invention relates to photocopiers, printers and facsimile methods and devices that employ a multi-beam light source to expose a photoconductive member by more than one laser beam according to an image data.
2. Description of the Related Art
Image forming apparatuses with multi-beam light sources are known, some of which provide a function of changeable writing density. In such devices, a laser diode and a beam polarizing unit are used for producing the multiple light beams. Beams are first produced, then processed in a beam polarizing unit and subsequently scanned across a photoconductive member. The beams form respective beam spots in a sub scanning direction, and thus the photoconductive member is exposed by more than one beam at the same time.
In above-mentioned image forming apparatus with the multi-beam source, a variable-pitch unit is provided for changing a pitch between beam spots in the sub-scanning direction on the photoconductive member. This variable-pitch unit has a beam generating unit that is rotated about an optical axis of the source of light. Also included are a motor for driving the variable-pitch unit and a position detecting unit for detecting a rotation of the beam generating unit.
FIG. 8 shows a conventional digital copier with a multi-beam light source. In particular, this digital copier has an image reading unit 11, a printing unit including a laser beam scanning unit 27 and an ADF (automatic document feeder) 27. The ADF 27 feeds documents to a contact glass 14 one sheet at a time and the sheet on the contact glass 14 is read by the image reading unit 11. When the reading operation is finished, the ADF 27 removes the sheet.
The image reading unit 11 has a first carriage, which has a document-illuminating lamp 15, a reflector 16 and a first mirror 17, and a second carriage, which has a second mirror 18 and a third mirror 19. When the image reading unit 11 reads an image of a document, the first carriage moves with a uniform speed and the second carriage follows the first carriage at half the speed of the first carriage.
By moving the first carriage and second carriage in this manner, an image of the sheet on the contact glass 14 is scanned. As part of the scanning operation, the document-illuminating lamp 15 and the reflector 16 illuminate the document on the contact glass 14, and a reflected light from the document is sent to a CCD (charge-coupled device) sensor 21 and the image of the document is formed on the CCD sensor 21 by way of the first mirror 17, second mirror 18, third mirror 19 and color filter 20.
The CCD sensor 19 converts the received optical signal of the image to an electrical signal and outputs the electrical signal as an electrical, analog signal. When a color CCD, which consists of three CCD lines (for red, green and blue, for example) is used, the image reading unit 11 can also read color documents.
After finishing the reading operation, the first carriage and the second carriage return to a home position. The analog image signal of the image of the document outputted from the CCD sensor 22 is converted to a digital image signal by an analog/digital converter. The resulting digital image signal is then processed, for example, by converting to a binary signal, converting to a multiple value, performing half toning, scaling and editing operations, at an image processing unit 23.
In the print unit 12, a photoconductive drum 25 rotates and is charged uniformity by a charging unit 26. The charged photoconductive drum 25 is exposed by a laser beam outputted from a laser beam scanning unit 27 and a electrostatic latent image is formed on the photoconductive drum 25 according to the digital image signal from the image processing unit 23. A developing unit 28 then develops the electrostatic latent image on the photoconductive drum 25.
A recording paper is fed from a paper feeder selected from among candidate paper feeders 33, 34 and 35. The recording paper from the selected paper feeder is fed to a register roller 36 and fed to the photoconductive drum 25 with a suitable timing for the developed image on the photoconductive drum 25 to coincide with the paper fed by the register roller 36. Accordingly, the developed image on the photoconductive drum 25 may then be transferred to the recording paper.
Next, the recording paper is separated from the photoconductive drum 25 by a separating unit 31 and fed to a fusing unit 38 by a feeder 37, and the image is fused on the recording paper by the fusing unit 38. After fusing, the paper is fed out of the unit and onto a tray 39. The photoconductive drum 25, after the separating operation, is cleaned and residual toner on the photoconductive drum 25 is removed by a cleaning unit 32.
FIG. 9 is a diagram of the laser beam scanning unit of the digital copier in FIG. 8. A semiconductor laser in a beam generating unit 40 generates a laser beam and the generated laser beam is changed to a parallel ray of light by a collimate lens in the beam generating unit 40 and shaped by an aperture in the beam generating unit 40. This shaped ray of light is compressed in the sub scanning direction by a cylindrical lens 41 and then enters a polygon mirror 42.
The polygon mirror 42 is a regular polygon that rotates with a uniform speed and predetermined direction, under control of a polygon motor 43 (FIG. 8). The rotation speed is decided according to a rotation speed of the photoconductive drum 25, a writing speed of the laser beam scanning unit 27 and a number of reflective surfaces of the polygon mirror 42. The laser beam provided by the cylindrical lens 41 is reflected and directed to an f.theta. lens 44.
In FIG. 9, the f.theta. lens 44 is shown to convert the laser beam reflected by the polygon mirror 42, which has constant angular speed, to a laser beam which has constant speed on the photoconductive drum 25. The laser beam from the f.theta. lens 44 reaches the photoconductive drum 25 by way of a reflective mirror 45 and a dustproof glass element 46.
Moreover, in an out of image area (i.e., a portion of the scanning region in which the image will not be formed), the laser beam from the f.theta. lens 44 is reflected by a synchronous detecting mirror 147 and reaches a synchronous detecting sensor 48. A synchronous signal that serves as a start timing trigger in the main scanning direction is generated according to a signal output from the synchronous detecting sensor 48.
FIG. 10 is a diagram of the LD (laser diode) unit in the laser generating unit 40. In FIG. 10, the LD unit 50 has a first semiconductor laser LD1, a second semiconductor laser LD2, a first collimate lens L1, a second collimate lens L2, 1/2 wavelength plate L3, a reflective surface L4, and a polarizing beam splitter L5.
A first laser beam, A, generated by the first semiconductor laser LD1 is converted into a parallel ray of light by the first collimate lens L1 and enters the polarizing beam splitter L5. A second laser beam, B, generated by the second semiconductor laser LD2 is converted into a parallel ray of light by the second collimate lens L2 and a polarizing surface of the second laser beam B is rotated by 1/2 WAVELENGTH plate L3. Next, the laser beam B is outputted with a prescribed angle so as to meet the first laser beam A by way of the reflective surface L4 and the polarizing beam splitter L5.
In this LD unit 50, by rotating the LD unit 50 about an optical axis of the first laser beam A, or the second laser beam B, a pitch of laser beam, a space between the first laser beam A and the second laser beam B about sub scanning direction, can be changed.
In this digital copier, the laser beam generating unit 40 rotates the LD unit 50 about the optical axis of the beam. FIG. 11a and 11b show this rotating system. FIG. 11a is a diagram of a top of view of the laser generating unit 40 in FIG. 9 and FIG. 11b is a diagram of a view of direction "C", as shown in FIG. 11a. In FIG. 11a, a bracket 2 that supports the beam generating unit 40 is set on a wall 27a of the laser beam scanning unit 27. The bracket 2 has a circular hole, and a cylindrical part 1a of a rotating unit 1 is set in the hole so as to rotate freely on the bracket 2. The wall 27a also has a circular hole, which is bigger than the hole in the bracket 2.
The rotating unit 1 can rotate freely on the cylindrical part 1a. A brim part 1b is set on the cylindrical part 1a of the rotating unit 1. Because one end of a spring 1c is fastened to the brim part 1b and another end of the sprint 1c is set around the cylindrical part 1a, the rotating unit 1 is pushed to the bracket 2.
In the rotating unit 1, there is the LD unit 50. An axis of rotation of the cylindrical part 1a is set in parallel with the axis of the first laser beam A or the second laser beam B. The first laser beam A and the second laser beam B are outputted from the cylindrical part 1a. A circuit board 3 for controlling the LD unit 50 is connected to the rotating unit 1 with a bracket 3a.
In FIG. 11b, there is a spring 47 between the rotating unit 1 and the bracket 2, and the rotating unit 1 is urged by a contracting force of the spring 47 so as to rotate. This rotating of the rotating unit 1 is stopped and controlled by the controlling component 4. The controlling component 4 is controlled by a stepping motor 5 which is set on the bracket 2 and the stepping motor 5 is driven by a command from a main board.
FIGS. 12a and 12b show the controlling component 4 and related components. A screw part 5a is set on the axis of the stepping motor 5 and the screw part 5a meshes with a female threaded screw in the controlling component 4.
FIG. 12b is a view of the components in FIG. 12a, from the direction D. A shape of the controlling component 4 is in the form of a "D" for blocking the rotation of the controlling component 4. An arresting rotation component 6 is connected to the stepping motor and the arresting rotation component 6 has a hole which is in the form of a "D". The controlling component 4 fits the hole of the arresting rotation component 6 and protrudes or recedes by rotating the stepping motor 5.
In reference to FIGS. 11a and 11b, when the stepping motor 5 rotates, the rotating unit 1 is rotated about the cylindrical part la because the controlling component 4 protrudes or recedes. By this rotation of the rotating unit 1, because the LD unit 50 on the rotating unit 50 is rotated also, the space between the first laser beam A and the second laser beam B about the sub scanning direction on the photoconductive drum 25 is changed.
There is a sensor interrupting part 1d which is in the form of a "L" on the rotating unit 1. The interrupting part 1d is inserted between a photo interrupter 7 and interrupts a light in the photo interrupter 7 by rotating of the rotating unit 1. By this interruption, it is detected that a position of the LD unit 50 is in a home position.
FIG. 13 shows exemplary rotations of the rotating unit 1 for different desired dot pitches. Previously, for changing a writing density by laser beam sub scanning direction, first, the rotating unit 1 is rotated to the home position. Next, the stepping motor 5 is driven according to an inputted predetermined number of steps and the LD unit 50 on the rotating unit 1 is rotated by the predetermined angle. For example, for changing from 400 dpi (dots per inch) to 600 dpi, first, the rotating unit 1 is rotated with angle "a" in FIG. 13, next, the rotating unit 1 is rotated with angle "b". In this changing, a rotating distance of (a+b) is needed.
It is preferable that this rotating distance is short so as to enable a quick density change. FIG. 14 is a diagram of characteristics of a stepping motor. If a high frequency signal is inputted for fast execution, a torque becomes small. Therefore, it is preferable that this rotating distance is short.
There is a backlash between the screw part 5a and the controlling component 4. Therefore, for accurately controlling the rotation angle of the rotating unit 1, it is needed that a positioning control is executed by one way rotating. In other words, if a positioning control is executed by a direction of uplift in FIG. 11b, a positioning control has to be executed by a direction of uplift on every occasion.
If a same number of steps is inputted to the stepping motor 5, there is difference of amount of rotating between the case of uplifting and moving down. For instance, in FIG. 15, in spite of the number of steps input for setting 400 dpi, the rotating unit 1 moves to a position Q in the case of moving down and a position P in the case of uplifting because of backlash .DELTA.X. Because there is the backlash .DELTA.X, an amount of rotating is not accurate and it is impossible that an accurate density change can be made.
This accuracy phenomenon occurs in the case of setting the home position. For correct density of writing by getting a correct home position, a positioning control is executed by one way rotating. In FIG. 16, the case is shown for changing from 400 dpi to 600 dpi, a rotation positioning control for the home position and for 600 dpi are executed by direction of uplift also. For this rotating to get a correct density, it is necessary that the amount of rotating "a" and .DELTA.X are executed as a substitute for the amount of rotating "a" in FIG. 13, and a positioning control to HP (home position) is executed by uplift. Next, a rotating to a position for 600 dpi is executed by uplift relative to the HP.
In this case, the amount of rotating is (a+.DELTA.X+.DELTA.X+b)=(a+b+2.DELTA.X). This amount of rotating (a+b+2.DELTA.X) is not preferable because the rotation distance is too long.
As described above, in a conventional digital copier with a multi-beam light source, because the amount of light source rotation for changing a writing density is long, a writing density change operation also takes a long time.